Age spots and petechiae are both causes of spots on the arms, says Mayo Clinic. Age spots are harmless, while petechiae may have minor or serious causes.
Age, or liver, spots most frequently occur after the age of 50 but also appear in younger individuals, reports Mayo Clinic. Age spots appear on skin that has frequent sun exposure and can also result from tanning beds. The spots are areas of high melanin concentration that appear brown or nearly black. They are benign and skin-lightening products cause them to fade over time. Protecting the skin during time in the sun with clothing or sunscreen prevents the appearance of age spots.
Petechiae are tiny red, brown or purple spots that often appear in clusters and are the result of burst or broken capillaries bleeding into the surrounding skin, says Mayo Clinic. Injuries, straining, sunburn, medications and some medical conditions can all cause petechiae to appear. Anticoagulants, atropine, naproxen, penicillin and quinine are among the medications known to sometimes cause petechiae. These spots can also result from a number of medical conditions, such as Rocky Mountain spotted fever, scarlet fever, strep throat, leukemia and scurvy. Because of possible associations with drug interactions and serious diseases, it is best to visit a doctor if petechiae appear without an apparent cause.